Herpes simplex virus (HSV) causes lifelong latent infections in humans. It is responsible for significant disease, ranging from cold sores and genital infections to neonatal infections, blindness and fatal encephalitis. The long-term goal of this project is to understand the molecular mechanisms that HSV uses to gain entry into host cells. Herpesvirus membrane fusion and entry is a complex cascade of interactions involving multiple viral glycoproteins and multiple cellular triggers. An emerging concept in herpesvirology is that the endosomal pH of the host cell is required for viral entry, often in a cell type specifi manner. The mechanistic role that low pH plays is not clear. We recently identified HSV envelope glycoprotein B (gB) as the principal target of endosomal pH. Mildly acidic pH reversibly alters the antigenic structure and oligomeric conformation of gB in vitro and during virus entry into cells. Based on these newly identified conformational changes, we propose to develop herpesviral entry inhibitors and to elucidate the role of gB in driving the fusion reaction In Specific Aim # 1, we will test the hypothesis that gB conformation change is a novel target for antiviral intervention. We will identify specific and potent peptide inhibitors of gB structural transition. Inhibitors will be assessed for the ability to block viral membrane fusion and entry. I Specific Aim # 2, we will define the contribution of the gB cytoplasmic tail to its conformation change and membrane fusion. Our experimental design employs techniques of cell biology, biochemistry, and molecular virology. HSV utilizes a pH-dependent endocytosis pathway in epithelial cells, the initial portal of entry in the human host. Thus, achieving these aims will delineate a key step in the mechanism of the initial infection of target cells and reveal a means to prevent it. The results will provide a mechanistic understanding of conformational change in gB and its relation to membrane fusion.